Fix DERP WebSocket URL to include non-standard ports.

Tailscale's derphttp client ignores DERPPort when building WebSocket
URLs, causing connections to fail when DERP servers run on non-443
ports (e.g. :8443). The TCP dial path correctly handles DERPPort
but the WebSocket path (used in WASM/browser builds) does not.

--- a/derp/derphttp/derphttp_client.go
+++ b/derp/derphttp/derphttp_client.go
@@ -279,10 +279,19 @@
 		return c.url.String()
 	}
 	proto := "https"
+	var port string
 	if debugUseDERPHTTP() {
 		proto = "http"
+		port = "3340"
 	}
-	return fmt.Sprintf("%s://%s/derp", proto, node.HostName)
+	if node != nil && node.DERPPort != 0 {
+		port = fmt.Sprint(node.DERPPort)
+	}
+	host := node.HostName
+	if port != "" {
+		host = net.JoinHostPort(node.HostName, port)
+	}
+	return fmt.Sprintf("%s://%s/derp", proto, host)
 }
 
 // AddressFamilySelector decides whether IPv6 is preferred for